Let me start by clarifying that I do not support Zoo's actively taking animals from their natural habitats for purposes of human entertainment, or profit.

However, having said that I will argue that a Zoo dedicated to the preservation of certain species of animals, particularly those who are endangered or threatened, seems to me the only option.

Currently, according to statistics (assuming they're accurate) 50% of the planet is wilderness (http://www.sciencedaily.com...), and seeing that within the next thirty five years the human population is to reach ten billion (http://www.un.org...) and that a quarter of all current species alive today will have gone extinct by that time (http://www.livescience.com...), I can see no other way in which to preserve the existence of many of the world's species than Zoo's.